1. Field of the Invention
The invention concerns a steam turbine which is operated with nozzle group control in the part load range.
2. Discussion of Background
Control wheels with separately opening nozzle groups are used for partial admission in steam turbine construction because the efficiency obtainable by this means over the important power range is better than that of other systems, the effect of the control wheel being to extract work from the steam in such a way that the power control is, in itself, optimal. In order to achieve full admission in the following stages, an equalizing space is provided to permit the transition from partial admission to full admission.
In the case of steam turbines in the small power class, which use by a small quantity of steam and have a small rotor diameter, the diameter of the control wheel is made larger than the diameter of the subsequent stages. In consequence, sufficient space is gained between the outlet plane of the partial admission control wheel and the inlet plane of the first full admission stage for the flow to become more or less evenly distributed over the whole cross-section of the flow duct between the control wheel and the entry into the following part of the turbine so that the losses due to inhomogeneous flow in the full admission stages remain small.
In steam turbines of larger powers which use large steam quantities and have large rotor diameters, however, it is not possible to design the control wheel so that it is much larger than the subsequent full admission part. In consequence, the space available between the partial admission control wheel and the subsequent full admission part of the turbine, for equalizing the flow over the complete periphery of the flow duct is smaller and flow inhomogeneities remain. An excessively small wheel space then leads to substantial losses in part load operation also. In the extreme case, the control wheel could be designed to have the same diameter as the blading of the following turbine stage; in this case, however, a special equalizing section would be necessary - in the form of a very large axial distance or a flow reversal, for example - which implies a lengthening and/or deterioration of the turbine. Another disadvantage in the case of flow inhomogeneities is that the blades of the stages following the control wheel can then be excited to undergo damaging vibrations.